1. Field of the Invention
The present invention relates generally to methods for depositing a coating on a substrate, and more particularly, to methods for depositing a coating by using a thermal spraying technique to form a coating having a garnet crystal structure phase.
2. Description of the Related Art
Ceramic materials having the garnet crystal structure, such as rare-earth aluminates, have been identified as potential materials in various industry applications, and have been of particular interest in industry areas ranging from semiconductor processing to aircraft and land-based turbine engine fabrication. In particular, garnet structured ceramic materials have been identified as having useful properties, including high-temperature strength, such as at elevated temperatures on the order of 1300° C. and higher. In addition, garnet ceramics are resistant to corrosion from reactive materials such halogens and acids. Such problems are especially acute in semiconductor processing. Silicon wafers are typically etched in reactors using halogen radicals that are extremely corrosive. It is important that the immediate environment of the wafer (which typically consists of chamber walls with some kind of lid and an electrostatic chuck to hold the wafer) not corrode rapidly, otherwise small particles of eroded material may fall on the wafer and create fatal defects.
As is understood in the art, it is generally difficult to fabricate large ceramic components from a uniform or monolithic ceramic composition. In particular, it is generally difficult to form large, dense ceramic components, as thermal stresses and non-uniformities tend to produce cracking in the component. Accordingly, techniques for depositing ceramic materials, including garnet structure ceramic materials, have been investigated. In this context, coatings of garnet ceramics generally play a role as thermal, chemical or mechanical barriers, which function to protect the underlying substrate.
Several techniques have been employed in the art to deposit garnet ceramics on substrates. In one such technique, raw materials including aluminum and yttrium raw materials are deposited using RF magnetron sputtering. However, this technique suffers from several disadvantages. For example, the sputtering technique cannot produce robust coatings having a thickness greater that about 5 microns, which thickness is generally desirable for corrosion protection. In particular, the process is slow and expensive, and thick coatings generally delaminate from underlying substrate due to internal residual stresses in the coating.
Another technique attempts to deposit yttrium aluminum garnet (known in the art as “YAG”), having the nominal composition Y3Al5O12 coatings by a thermal spray process. In this technique, liquid precursors including suspensions of elemental or compounds of yttrium and aluminum are fed to an RF induction plasma torch, and thermally sprayed to form a coating. Here, the coating as-sprayed was found to have the perovskite phase, and a subsequent high temperature exposure was executed to convert the perovskite phase to the garnet phase.
While the foregoing thermal spray process provides significantly higher deposition rates than the above-described sputtering process, the secondary heat treatment step to effect phase transformation of the deposited coating is problematic. In particular, the temperatures at which the heat treatment step is executed have a tendency to damage or affect the structural integrity of certain types of underlying substrates such as aluminum-based alloys.
Accordingly, the present inventors have recognized a need in the art to provide improved processes for forming garnet ceramic coatings.